Certain embodiments of the present invention generally relate to an x-ray system capable of varying the distance between the image receptor and the x-ray source. Certain embodiments of the present invention generally relate to a mobile C-arm based x-ray system that varies the source to image distance while acquiring images for three dimensional (3D) image reconstruction.
3D x-ray imaging has become increasingly useful in medical diagnostic procedures and surgical planning. Computerized Tomography (CT) was the first type of system used for these purposes. Conventional CT systems employ a fan-shaped x-ray beam directed at a detector array which has a width dimension much smaller than its length. To obtain complete scans of a significant volume of anatomy, the x-ray tube and detector array are rotated around the patient a number of times while the patient is advanced along the axis of rotation.
More recently, area-beam detectors, such as image intensifiers, have been employed in acquiring 3D image data. Such systems are based on conventional x-ray systems used for cardiovascular and/or surgical interventional imaging. The advantage to such systems is that they can acquire a full 3D dataset faster than a CT system, and thereby better capture dynamic events. For example, area-beam 3D imaging of the vessels of the brain using contrast agents has become extremely in the diagnosis and treatment of intra-cranial aneurysms.
Heretofore, area-beam detector 3D x-ray imaging systems have operated by rotating an x-ray tube and detector in circular paths around a central axis of rotation. The axis of rotation is positioned to be at the center of the region or volume of interest of the patient anatomy. The x-ray source and x-ray receptor, usually an image intensifier, are typically mounted on opposite ends of a rotating C-arm support assembly. The x-ray source irradiates a patient with x-rays that impinge upon a region of interest (ROI) and are attenuated by internal anatomy. The attenuated x-rays emerge from a back-side of the patient and are incident upon the receptor. 3D image data is acquired by taking a series of images as the x-ray tube/C-arm/receptor assembly is rotated about the axis of rotation on which the region of interest within the patient is centered.
Conventional mobile C-arm assemblies utilize simple support structures and geometries to mount the x-ray source and the receptor on the C-arm. The support structure holds the x-ray source and receptor on the C-arm and maintains a predetermined, constant distance between the x-ray source and receptor. Hence, the distance between the x-ray source and the axis of rotation, and the distance between the receptor and the axis of rotation remain constant and fixed.
However, conventional mobile C-arm assemblies experience certain problems when using an x-ray source and receptor fixedly mounted on a C-arm to generate 3D reconstruction images. The 3D reconstruction images are formed for an ROI located within a patient having an oblong cross-section (e.g., a patient laying face up on a table may have a longer width from shoulder to shoulder and a shorter height from front to back).
FIGS. 4-7 illustrate a conventional C-arm assembly 300 that rotates about a circular path 312. The radius of the circular path 312 must be large enough to pass the widest portion of the patient""s anatomy (e.g., shoulder to shoulder). Hence, during a set-up operation before acquiring a series of patient images, the patient is positioned between the image receptor 306 and x-ray source 304 to prevent the x-ray source 304 or image receptor 306 from contacting the patient during any part of the rotational scan. In order to obtain patient images, the x-ray source 304 and image receptor 306 are rotated to various scan angles about the patient. Each scan angle has a corresponding trajectory through the ROI. As the scan angle varies, the trajectory between the image receptor 306 and x-ray source 304 similarly varies and, in addition, the distance between the image receptor 306 and the exit surface of the patient 308 varies considerably. The distance also varies between the x-ray source 304 and entrance surface of the patient 309.
As shown in FIGS. 4 and 5, the foregoing phenomenon result in a conventional C-arm assembly 300 affording a limited 3D reconstruction volume 302. The C-arm assembly 300 operates in an ISO centered manner in which the ROI within the patient remains centered within the x-ray field 303. In order to maintain the ROI centered within the x-ray field 303, the x-ray source 304 and image receptor 306 rotate about the patient along a circular arc 312. The patient is oblong shaped and thus as the x-ray source 304 and image receptor 306 rotate about a circular path, the image receptor 306 rotates from positions proximate the patient to positions remote from the patient. When the image receptor 306 is located remote from the patient exit surface 308, the area of anatomy that can be imaged is limited by the geometric magnification (generally denoted by the arrow 310). Therefore, the dimensions of reconstruction volume 302 are limited as well.
Also, as shown in FIG. 6, the image receptor 306 is positioned a distance 314 from the patient exit surface 308. The x-rays are emitted from the x-ray source 304 in a cone beam shape and thus as the image receptor 306 is moved further from the patient, the x-ray field 303 expands. Expansion of the x-ray field 303 effectively magnifies at the image receptor 306 each image obtained from the region of interest. As the distance 314 increases, the amount of magnification greatly increases, particularly for images taken at scan angles associated with the shortest distance through the anatomy (e.g., front to back in the example of FIGS. 4-7). The large amount of magnification associated with positioning the image receptor 306 a distance 314 remote from the patient exit surface 308 causes the image to be blurred, due to the large projected penumbra of the focal spot. As the sharpness of the focal spot decreases, the quality of the 2D image data decreases, and therefore the quality of the 3D reconstructed data decreases.
Further, as shown in FIG. 7, the distance from the x-ray source 304 to the image receptor 306 remains constant. However, as the distance 316 decreases, the image receptor 306 is positioned farther away from the patient in some views, while the x-ray source 304 is positioned closer to the patient""s skin than necessary at certain scan angles, giving rise to unnecessarily high irradiation doses.
Hence, a need remains for an improved x-ray imaging system capable of reconstructing 3D volumes of patient information for a region of interest that overcomes the problems above and previously experienced.
In accordance with at least one embodiment, a medical diagnostic imaging system is provided. The system has a C-arm unit that moves an image receptor and x-ray source around a patient in non-circular arcs. The receptor and x-ray source are supported by a support structure, and at least one of the receptor and x-ray source are moveable upon the support structure, such as in a radial direction toward and away from a central axis of the support structure. The distances between at least one of the patient and source, and the patient and receptor are varied such that the receptor and/or source remain positioned within a desired distance from the patient""s surface.
In accordance with at least one embodiment, the medical diagnostic imaging system includes an image processor. The image processor collects image exposures at exposure positions as the receptor and x-ray source are moved around the patient. Position data corresponding to the exposure positions is collected and used, together with the image exposures, to construct a three dimensional volumetric data set. Images are displayed based upon the three dimensional volumetric data set.